Gypsum is commonly manufactured by drying, grinding, and calcining natural gypsum rock. The drying step involves passing crude gypsum rock through a rotary kiln to remove free moisture. The dried rock is then ground. The calcination is to dehydrate the ground gypsum rock to hemihydrate:CaSO4.2H2O+heat→CaSO4.½H2O+3/2H2O
Gypsum reacts with water and sets rather quickly. The setting is a reversal of the calcination. The hemihydrate gypsum dissolves in water until it is saturated. The soluble hemihydrate forms dihydrate which is less soluble than hemihydrate. As dihydrate crystals precipitate, the hemihydrate solution is no longer saturated, and the process continues to consume the hemihydrate gypsum. The reaction can be followed by measuring the heat evolved. Initially there is very little reaction and no rise in temperature. This time is referred to as the induction period. As the amount of dihydrate increases, the mass thickness increases, and the gypsum sets.
A large amount of excess water is generally required to provide the gypsum slurry with sufficient fluidity during the process. It is desirable to reduce the amount of excess water to save energy and production cost associated with water removing.
Water reducing agents are known. For example, naphthalene sulfonate has been used as a water reducing agent. One drawback of naphthalene sulfonate is that it requires a high dosage. Further, gypsum compositions containing naphthalene sulfonate tend to set too fast. To slow down the setting, a setting retardant is often added.
Recently, acrylic-polyether comb-branched copolymers are used as water reducing agents in gypsum compositions. See U.S. Pat. No. 6,527,850. The comb-branched copolymers can be used in much lower dosages than naphthalene sulfonate. However, the comb-branched copolymers tend to retard the setting.
In sum, new water reducing agents are needed. Ideally, the water reducing agents would not only improve the fluidity but also have no or reduced retardation on gypsum setting.